1. Field of the Invention
The field of the invention is data processing, or, more specifically, a wide-bandwidth linear regulator and a method of regulating a power supply.
2. Description of Related Art
A voltage regulator is an electrical circuit designed to maintain a constant voltage level at its output even as operating conditions change over time. Every electronic circuit is designed to operate off of some supply voltage, which is usually desired to be constant. A voltage regulator provides this constant DC output voltage and contains circuitry that continuously holds the output voltage at the desired value regardless of changes in load current or input voltage (this assumes that the load current and input voltage are within the specified operating range for the regulator). We address the issue of voltage regulation, specifically, on-chip regulation of potentially noisy external power supplies to create a high DC accuracy, low AC noise voltage level that is used to power some local circuitry. Maintaining accurate voltage regulation is particularly challenging when the load current variations are sudden and extreme, e.g. minimum load to maximum load demand in a short period of time. Such sudden and extreme variations in load current can occur in applications in which portions of the circuitry being powered by the regulator switches from an idle state to a state with high activity factor (maximum workload).
Linear regulators are the most commonly used voltage regulator type in integrated circuits (ICs) and have a number of advantages. They can be integrated, requiring no off-chip components such as inductors. Unlike switching types, linear regulators generate no inherent ripple of their own, so they can produce a very “clean” DC output voltage, achieving low noise levels with minimal overhead (cost). Typically, a linear regulator operates by modulating the voltage drop across a series pass element, which can be modeled as a voltage-controlled resistance. The control circuitry monitors (senses) the output voltage. If the output voltage is lower than desired, a voltage is applied to the series pass element which decreases its resistance; since less voltage is dropped across the series pass element, the output voltage rises. Similarly, if the output voltage is higher than desired, the resistance of the series pass element is increased, so more voltage is dropped across the series pass element, and the output voltage falls. Since the output voltage correction is achieved with a feedback loop, some type of compensation is required to assure loop stability which can slow the feedback response of the regulator. Hence, any linear regulator requires a finite amount of time to correct the output voltage after a change in load current demand. This “time lag” defines the characteristic called transient response, which may not be fast enough for applications with sudden and extreme load current variations, such as the circuit application referenced above. To minimize this time lag, generally the linear regulator's bandwidth is increased. However, the need to maintain adequate loop stability (phase margin) limits the achievable bandwidth of most linear regulators. Filtering of regulated voltage domains with decoupling networks and parasitic device capacitances form time constants called ‘poles’ which cause accumulated phase shift in the regulator's open loop response. Such accumulated phase shift in the open loop can cause ringing or even oscillation at the linear regulator's output as the net phase approaches 180 degrees. Hence, to obtain both stability and fast transient response, the linear regulator's topological structure must provide a means to mitigate the problematic accumulation phase shift in the regulator's feedback loop when the open loop gain is greater than unity. Otherwise, the feedback error signal will become regenerative and cause instability in the loop.
To illustrate this point, FIG. 1 sets forth a Bode diagram illustrating frequency and phase response of an example linear regulator illustrated in FIG. 2. The example linear regulator of FIG. 2 includes a series pass element (230), a capacitance load element (220), a resistance load element (222), and an amplifier (228) with an input gate impedance (232). According to first-order principles, those skilled in the art will understand that a first pole (120) and a second pole (122) are formed in the feedback loop. The first pole (120) is formed due primarily to the impedance of the load of the regulator of FIG. 2, specifically the load capacitance element (220) and the load resistance element (222). The second pole (122) is formed due to the output impedance (226) of the amplifier (228) and the input gate capacitance (232) of the series pass element (230). Those skilled in the art will further understand that in the frequency domain, each pole will eventually contribute 90 degrees of phase shift to the open loop phase response. In order to maintain the unconditional stability of the negative feedback loop and satisfy the stability criteria, the open loop gain must be reduced below unity prior to the accumulated open loop phase reaching 180 degrees of phase shift. If the load capacitance element (220) is large, the first pole (120) will generally be low in frequency. However, any change in the load resistance element (222) will shift the first pole (120) up or down in frequency. Likewise, if the gate capacitance element (232) is large because the series pass element (230) is large, the second pole (122) will be generally be low in frequency. However, any change in the output impedance (226) will shift the second pole (122) up or down in frequency. If both the first pole (120) and the second pole (122) form essentially at the same frequency, the phase contribution from each will quickly accumulate phase shift in the open loop response. Furthermore, as illustrated in FIG. 1, if the open loop gain has not been attenuated to less than unity prior to the open loop phase accumulation reaching 180 degrees, loop oscillation will result.